This document describes a vehicle brake system for a land vehicle, wherein the vehicle brake system comprises an accumulator for accommodating hydraulic fluid. The accumulator accommodates a hydraulic fluid reserve volume in a non-actuated initial state of the vehicle brake system.
Vehicles typically have vehicle braking equipment in which, for braking, a hydraulic fluid is guided into the wheel brakes of the vehicle. However, vehicles increasingly also have an electrical machine which serves to drive the vehicle, in a manner at least assisting an internal combustion engine. The electrical machine has the advantage over an internal combustion engine that it can be operated in the manner of a generator. During operation of the electrical machine in the manner of a generator, the kinetic energy of the vehicle is converted into electrical energy. In this context, the recovery of energy from the kinetic energy of the land vehicle by means of the electrical machine can be called regeneration.
Whereas, in normal brake operation, kinetic energy of the vehicle is converted into thermal energy because of the frictional interaction between the brake blocks and brake discs on the wheel brakes, in the case of regeneration the kinetic energy of the vehicle is used to charge up the vehicle battery. The chemical energy that is stored in the charged battery is then used again during motive operation of the electrical machine, to drive the land vehicle. The vehicle is braked by means of the electrical machine, with the result that the wheel brakes can remain unactuated. Typically, this is indeed desirable, since any kinetic energy converted into thermal energy at the wheel brakes is no longer available for charging up the vehicle battery.
By comparison with a purely hydraulic braking procedure, however, building up the generative braking moment frequently takes somewhat longer. The braking action of the electrical machine that is operated in the manner of a generator also falls off at low speeds. For this reason, typically the electrical machine cannot brake the vehicle to a standstill and/or keep the vehicle reliably at a standstill.
For this reason, there is a concern that, during regeneration too, a hydraulic braking moment should first be built up and then the hydraulic braking moment should be replaced by a generative braking moment. Further, there are concerns that when speed falls below a minimum value, that is to say at the end of regeneration, hydraulic braking moment should be built up. So that the usual brake feel can continue to be imparted to the driver—that is to say so that there are no unexpected pedal reactions and there is no need for the driver to actuate the brake pedal again in the end phase of regeneration—hitherto there has been a need for a plurality of valve switching procedures and for a hydraulic pump to be driven. Because of the noises that occur during this, the driver may perceive the switching procedures in particular of the valve arrangements as disruptive.
For this reason, it would be desirable for as little noise as possible to be created when this function is implemented. Further, it would be desirable for the vehicle brake system to be suitable for purely hydraulically braked vehicles and for generatively braked vehicles, or at least to be capable of being adapted to the respective requirements relatively simply.